Reducing smog refers to air pollution episodes characterized by high concentrations of sulfur dioxide and smoke (or particulate aerosols). Reducing smog is also sometimes called London-type smog, because of famous incidents that occurred in that city during the 1950s.
Reducing smogs first became common when industrialization and the associated burning of coal caused severe air pollution by sulfur dioxide and soot in European cities. This air pollution problem first became intense in the nineteenth century, when it was first observed to damage human health, buildings, and vegetation.
There have been a number of incidents of substantial increases in human illness and mortality caused by reducing smog, especially among higher-risk people with chronic respiratory or heart diseases. These toxic pollution events usually occurred during prolonged episodes of calm atmospheric conditions, which prevented the dispersion of emitted gases and particulates. These circumstances resulted in the accumulation of large atmospheric concentrations of sulfur dioxide and particulates, sometimes accompanied by a natural fog, which became blackened by soot. The term smog was originally coined as a label for these coincident occurrences of atmospheric pollution by sulfur dioxide and particulates.
Coal smoke, in particular, has been recognized as a pollution problem in England and elsewhere in Europe for centuries, since at least 1500. Dirty, pollution-laden fogs occurred especially often in London, where they were called "pea-soupers." The first convincing linkage of a substantial increase in human mortality and an event of air pollution was in Glasgow in 1909, when about 1,000 deaths were attributed to a noxious smog during an episode of atmospheric stagnation. A North American example occurred in 1948 in Donora, Pennsylvania, an industrial town located in a valley near Pittsburgh. In that case, a persistent fog and stagnant air during a four-day period coupled with large emissions of sulfur dioxide and particulates from heavy industries to cause severe air pollution. A large increase in the rate of human mortality was associated with this smog; 20 deaths were caused in a population of only 14,100. An additional 43% of the population was made ill in Donora, 10% severely so.
The most famous episode of reducing smog was the so-called "killer smog" that afflicted London in the early winter of 1952. In this case, an extensive atmospheric stability was accompanied by a natural, white fog. In London, these conditions transformed into a noxious "black fog" with almost zero visibility, as the concentrations of sulfur dioxide and particulates progressively built up. The most important sources of emissions of these pollutants were the use of coal for the generation of electricity, for other industrial purposes, and to heat homes because of the cold temperatures. In total, this smog caused 18 days of greater-than-usual mortality, and 3,900 deaths were attributed to the deadly episode, mostly of elderly or very young persons, and those with preexisting respiratory or coronary diseases.
Smogs like the above were common in industrialized cities of Europe and North America, and they were mostly caused by the uncontrolled burning of coal. More recently, the implementation of clean-air policies in many countries has resulted in large improvements of air quality in cities, so that severe reducing smogs no longer occur there. Once the severe effects of reducing smogs on people, buildings, vegetation, and other resources and values became recognized, mitigative actions were developed and implemented.
However, there are still substantial problems with reducing smogs in rapidly industrializing regions of eastern Europe, the former Soviet Union, China, India, and elsewhere. In these places, the social priority is to achieve rapid economic growth, even if environmental quality is compromised. As a result, control of the emissions of pollutants is not very stringent, and reducing smogs are still a common problem.